This invention relates to the fixation of anionic materials, e.g., radioactive anionic species, with a complexing agent immobilized on a support such as a silicate glass or silica gel or charcoal matrix.
A number of radioactive isotopes are present in the cooling, operational and waste water from the daily operation of nuclear power plants and fuel rod holding tanks. While these radioactive isotopes are present in the water in very low concentrations, they are nonetheless highly radioactive and toxic to human life. Safe disposal or re-use of the contaminated water can only be conducted if a sufficient quantity of radioactive isotopes is removed to reach permissible levels.
The radioactive isotopes present in contaminated nuclear reactor water include cations and anions. The removal of radioactive cations using a porous glass cation exchanger is disclosed in co-pendinq application Ser. No. 370,437 filed Apr. 21, 1982, now U.S. Pat. No. 4,469,628, which is a continuing application of Ser. No. 39,595 filed May 16, 1979, now abandoned, which is a continuing application of Ser. No. 959,222, filed Nov. 9, 1978, now abandoned, each by Simmons, Simmons, Macedo and Litovitz and each entitled "Fixation By Ion Exchange of Toxic Materials In A Glass Matrix".
The anions present in solution consist primarily of I.sup.131 which has a half-life of 8 days but which possesses a significant threat to life due to its affinity for and high reconcentration in animal and human metabolic processes. After three months, the majority of the non-metal anions have generally decayed to stable isotopes; however, many of the longer-lived metal isotopes form anionic complexes such as chromates, cerates, and molybdates, which remain radioactive for longer time periods.
Radioactive isotopes also are formed in the vapors given off by various processes in the nuclear fuel cycle. Thus, there also exists a need in the art for cleaning these vapors.
Organic anion resins are typically used for decontaminating water used in nuclear reactors. However, they are readily decomposed by radioactivity, they cannot be dried, they are not readily compatible for use in mixed beds with the new types of glass cation exchangers coming on the market, and they cannot be put into a long-term chemically stable form, thus causing a serious danger to the environment through premature release of the radioactive isotopes.
U.S. Pat. No. 4,333,847 by Tran, Macedo, Simmons, Simmons and Lagakos entitled "Fixation By Anion Exchange of Toxic Materials In A Glass Matrix" discloses the use of a porous glass medium as an anion exchanger. The glass anion exchanger of the Tran et al patent, the disclosure of which is expressly incorporated herein by reference, is a porous silica glass or gel containing silica having interconnected pores. Non-radioactive cationic polyvalent metals such as zirconium or lead can be bonded to silicon of the glass or gel through divalent oxygen linkages on the internal surfaces of the pores. Non-radioactive anions such as hydroxyl groups are ionically bonded to the cationic metals and displaceable by the radioactive anions to provide a distribution of radioactive anions internally bonded within the pores of the glass or gel.
In an article by Amphlett et al, entitled "Synthetic Inorganic Ion-Exchange Materials--II. Hydrous Zirconium Oxide and Other Oxides," J. Inorg. Nucl. Chem., Vol. 6, pp. 236 to 245 (1958), hydrous oxides, such as hydrous zirconium oxide, are disclosed as anion exchangers in acid and neutral solution and as cation exchangers in alkaline solution. However, there is no teaching or suggestion in the Amphlett et al article of binding the hydrous metal oxide to the silicon atoms of a porous glass or a porous silica gel through divalent oxygen linkages and reacting the resulting product with radioactive or toxic anions.
British Patent Specification No. 1,363,491 by Wilhelm et al entitled "Recovery of Iodine and Iodine Compounds" discloses a sorbing material for the removal of molecular iodine and/or organic iodine compounds which have up to six carbon atoms from gases and/or vapors. The sorption agent comprises "amorphous silicic acid" impregnated with a metal salt such as silver nitrate. Thus, this patent specification also discloses other prior art processes for the removal of iodine.
As discussed in co-pending application Ser. No. 370,437, the disclosure of which is expressly incorporated herein by reference, water bodies have been contaminated in the past with mercury, cadmium, thallium, lead and other heavy metal cations. The concentration of the cations in the waste streams is very low thus presenting the problem of treating large volumes of water containing small amounts of toxic cations. These waste streams can be purified by ion-exchanging the poisonous cations into a porous glass or silica gel. The porous glass or silica gel contains at least 75 mol percent silica and has interconnected pores. Non-radioactive alkali metal cations, Group I(b) metal cations, or ammonium cations are bonded to silicon of the glass or gel through divalent oxygen linkages on the internal surfaces of the pores. The non-radioactive cations are displaceable by the heavy metal cations to provide a distribution of the heavy metal cations internally bonded within the pores of the glass or gel.
British Pat. No. 1,389,905 describes a process for the preparation of radioactive molybdenum-99 which comprises: (1) irradiating a uranium material to produce nuclear fission therein; (2) dissolving the irradiated uranium material in an aqueous inorganic acid to form a solution; (3) precipitating molybdenum-99 by contacting the resultant acid solution with alpha-benzoinoxime; (4) recovering and dissolving the molybdenum precipitate in an aqueous alkaline solution; (5) contacting the alkaline solution containing the molybdenum-99 with at least one adsorbent for the selective removal of impurities, the adsorbent being (i) silver-coated charcoal, (ii) an inorganic ion adsorbent or (iii) activated carbon; (6) thereafter recovering radioactive molybdenum-99. This patent also describes a process for the preparation of a highly pure radioactive molybdenum-99 having a high specific activity which comprises: (1) irradiating uranium oxide to produce nuclear fission therein, the uranium oxide being deposited on the inner walls of a sealed stainless steel cylindrical target, (2) dissolving the irradiated uranium oxide in an aqueous mixture of sulfuric and nitric acids to form a solution, (3) adding to the resultant acid solution a stabilizing amount of sodium sulfite and hold-back carrier amounts of ruthenium chloride and/or sodium iodide, (4) precipitating molybdenum-99 by contacting the stabilized solution with alpha-benzoinoxime, (5) recovering the precipitated molybdenum by dissolving in an aqueous sodium hydroxide solution, (6) contacting the resultant alkaline solution with silver-coated charcoal, (7) acidifying the resultant alkaline solution, adding an oxidizing agent and repeating steps (4) and (5), (8) contacting the sodium hydroxide solution with silver-coated charcoal and zirconium oxide, (9) contacting the sodium hydroxide solution with activated charcoal, and (10) recovering molybdenum-99. In this process, the adsorbents (inorganic ion exchangers such as zirconium oxide, charcoal coated with metallic silver, activated carbon) are used to remove impurities such as iodine and ruthenium from the molybdenum-containing solution, which is made strongly alkaline with potassium hydroxide. The decontamination of near-neutral solutions such as reactor coolant or effluent streams is not discussed.